vt(4): If the cursor didn't move, don't mark its position as dirty

Currently, this has no effect, because the cursor is always redrawn
anyway. But this will be useful after improvements to the vd_bitbltchr_t
callback API.

The vt_device structure members used to store the position of the cursor
as of the last redraw are renamed from vd_mdirty{x,y} to vd_mold{x,y}.
The associated comment is fixed too. Also, their value is now expressed
in pixels, not in character columns/row.

MFC after:	1 week
vt(4): Mark the current cursor position as dirty

Like r270273, this has no effect for now, because the cursor is always
drawn. This is in preparation of future changes to vd_bitbltchr_t API.

MFC after:	1 week
Use __FBSDID() properly.

Suggested by:	pluknet@
MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation
Add guards to ptrdiff_t definition in include/stddef.h

Back in 2011 obrien has added the #define macro in sys/sys/stddef.h to
guard ptrdiff_t. Add similar protection to the identical code in
include/stddef.h.

Submitted by:   Mariusz Zaborski <oshogbo@FreeBSD.org>
MFC after:      1 week
vt(4): Mark cursor position as dirty when we enable/disable it

MFC after:	1 week